Baking normal for high poly sculpture not applying to low poly mesh
Bake from multires only bakes the information of the multires onto the same object. Since you used the decimate modifier, you probably changed, if not destroyed the UVs. That's not the option you need. You need to bake the normals with the option Selected to Active checked, with your highpoly selected and your lowpoly active and slightly inside the high poly. Do check the UV map of your low poly one though.
Also: Make sure to check the normals in rendered or shaded view, your screenshot is a bit too low res to really make out what's going on.